Re: relation between Test Set - test Cases - Test fold
As Brendan says the Database Reference document is available within the QC application itself, from the Help > Documentation Library link. The information comes in a help file format (.chm) which some people are unable to open from the QC server. If that is the case for you, download the file to your workstation and open it there.
The actual data in the tables can be viewed through the Site Admin database, but drilling down into specific projects.
(Opinions and information contained in this post are wholly my own and do not reflect the opinions of my employer.)